When a data stream is transmitted between a sending device and one or more receiving devices, over a network, data of the data stream are liable to be lost
In the case of a packet-switched communication network, the data stream is cut up into network packets. On transmitting the data stream so cut up, network packets may get lost.
The receiving device is adapted to detect the network packets lost on transmission of the data stream.
Once the receiving device knows the lost network packets, it requests the sending device to retransmit the lost network packets.
If the number of these lost network packets to retransmit is high, the network may be congested.
One solution for avoiding congestion of the network consists of requesting the retransmission of certain lost network packets instead of requesting the retransmission of all the network packets. The lost network packets to request are determined according to the importance of the network packet.
The importance of a network packet is evaluated according to the visual impact of a lost network packet on an image.
The visual impact depends on the quantity of lost data, that is to say that if a first lost packet contains a higher quantity of data than a second lost packet, the visual impact on the image of the first lost packet is greater than the visual impact of the second lost packet. Thus, the first lost packet is considered as more important than the second lost packet.
Thus, a lost packet containing a high quantity of data is considered important since its loss risks causing a high visual impact on the image to decode, and thus degrading the quality of the image.
For example, FIG. 1 represents the network packets 1 corresponding to a video image 2. Each network packet 10, 11 comprises the data necessary for the decoding of a part of the image 2. On coding the video image 2, the image 2 has been cut up into several parts 20, 21 and each part has been compressed using a compression algorithm. Thus, a first image part 20 (named for example “slice” in MPEG language) is compressed and forms a first network packet 10, and a second image part 21 is compressed and forms a second network packet 11.
The sizes of the network packets 10, 11 are often almost the same. Nevertheless, the area of the first network packet 10 on the image has a higher value than the area of the second network packet 11.
Consequently, if the first and second packets 10, 11 are lost, the visual impact of the first network packet 10 on the image 20 is greater than the visual impact of the second network packet 11. That is to say that the first network packet 10 is considered as more important than the second network packet 11.
The receiving device knows the importance of each packet corresponding to an image, only once the image has been decoded.
Thus, once the importance of the packets is known, the receiving device can request the retransmission of the packets considered as important to have an acceptable image quality.
Nevertheless, the fact of decoding the image before requesting the retransmission of the packets considered to be important gives rise to a non negligible delay between the reception of the packets and the request for retransmission of the lost packets, and may have very considerable consequences on certain video applications.